The present invention relates to a feed system for feeding a rocket engine with at least one liquid propellant, the system including at least one feed circuit.
In the field of liquid propellant rockets, the term “POGO effect” is used to designate the liquid propellant in the feed circuit of the rocket engine entering into resonance with mechanical oscillations of the rocket. Since the thrust of a rocket engine varies with the rate at which propellant is delivered by the feed circuit, such an entry in resonance can give rise to rapidly diverging oscillations and can thus give rise to difficulties in guidance, and even to damage that may go as far as total loss of the payload or even of the vehicle. The term “POGO effect” does not come from an acronym, but rather from pogo stick toys comprising a rod with a spring that bounces in a manner that reminded technicians of the violent longitudinal oscillations of rockets, as caused by this effect. From the beginning of the development of liquid propellant rockets, it has therefore been very important to take measures to suppress this POGO effect. In the context of the present description, the term “suppress” is used to cover both total elimination and partial reduction.
Two main types of POGO effect corrector systems are known to the person skilled in the art: passive systems and active systems. With passive systems, the hydraulic resonant frequencies are changed so that they cannot coincide with the mechanical resonant frequencies of the rocket. They can also be damped. This is done, for example, by installing hydraulic accumulators in the propellant feed circuit. Such a hydraulic accumulator is normally formed by a pressurized volume containing both gas and liquid, which volume is in communication with the feed circuit. The hydraulic accumulator operates as a mass-spring-damper system in which the mass is the mass of liquid in the accumulator, the spring is formed by the gas, and the damping comes from the viscosity of the liquid entering and leaving the accumulator via a narrow duct. French patent application FR 2 499 641 discloses one example of such a hydraulic accumulator that is adjustable in order to enable it to be adapted to different rocket engines. Nevertheless, variation in the compressibility and damping parameters of that accumulator cannot be carried out while the rocket engine is in operation. Another “passive” method of correcting the POGO effect consists in changing the hydraulic resonant frequency of the feed circuit by injecting a fixed flow rate of gas into the circuit so as to change the speed of sound in the circuit. In contrast, with active systems, an opposing oscillation of pressure and flow rate is established in the feed circuit to counter the oscillations that are measured in the circuit.
Nevertheless, both passive systems and active systems present drawbacks. Passive systems are not appropriate for rockets that present great variability in their mechanical frequencies while they are in operation, since they do not damp modes outside a narrow band around the frequencies for which they are designed. In the event of there being a difference between the predicted dynamic behavior and the real dynamic behavior of the flight of the rocket, they are not in a position to correct their action. Active systems are liable to have positive effects only locally and they can also generate effects that are negative, whether local or global.
In Japanese patent application JP 03-287498 A, there is proposed a POGO effect corrector system with an adaptive hydraulic accumulator. In that adaptive hydraulic accumulator, the compressibility can be varied by varying the pressure of the gas, and above all the damping can be varied by varying a flow-restriction device. Nevertheless, that presents several drawbacks. Firstly, varying pressure serves to vary compressibility in the hydraulic accumulator in only a very restricted manner. In addition, although it is possible to control greater variation in damping, the variable restriction device has moving parts in the flow of propellant, and that can lead to problems of reliability, particularly if the propellant is cryogenic.